One method of producing glass fibers is to attenuate molten galss through a precious metal bushing to produce fibers. The bushing forms a container with holes spaced therein through which the molten glass is drawn by mechanical means. It is advantageous to heat this bushing to produce a uniform temperature for drawing the molten glass. A high electrical current is passed through the metal to electrically heat the bushing. The diameter of the fibers produced is dependent upon the composition of the glass, the temperature of the glass, the temperature of the bushing, the thermal conditions below the bushing which affects the rate of cooling of the molten glass fibers, and the stress introduced into the fiber by the mechanical attenuation of the fibers. The object of the attenuation process is to produce a plurality of fibers of uniform diameter. In large multiple segment bushings it has been found that maintaining a constant temperature within each segment of the bushing aids in maintaining uniform fiber diameter.
Accurate determination of temperature in a glass fiber forming bushing is in the range of 2000-2500 degrees Fahrenheit. Thermocouples have a short life at this temperature, they only measure temperature at one point and there is a finite time lag between a change in temperature and a change in thermocouple reading. Infrared temperature measurement has been frustrated because of the presence of the issuing streams of molten glass and the crowded conditions beneath the bushing caused by fin shields and other devices. An accurate method of determining the average temperature across a segment of a multiple segment bushing can be accomplished by determining the instantaneous resistance change in each segment of the bushing.
The multiple segment bushing acts as a series connection of resistors whose resistance changes with temperature. Over a narrow band of temperatures, within the glass melting range, the value of each resistor changes in a linear fashion with temperature. This linear function can be expressed as: EQU R.sub.i =R.sub.o [1+.alpha.(T.sub.i -T.sub.o)]
where R.sub.o is the resistance of the segment of the bushing at a set point temperature T.sub.o, and R.sub.i is the instantaneous resistance of the i th segment of the bushing at the instantaneous temperature T.sub.i. The term .alpha. is the temperature coefficient of resistance for the material of the bushing and is given in various reference books of various metals and temperature ranges. The current flowing in a bushing and the voltage drops across each segment of the bushing can be measured. By using ohm's law, the instantaneous resistance can be determined. The resistance at a set point temperature of the material, R.sub.o, being known, and the coefficient of resistance for temperature, .alpha., being known the change in temperature can be directly inferred. This signal can then be used for control and balance of all segments of the bushing.